Structural basis of activation of mammalian heme peroxidases

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Structural basis of activation of mammalian heme peroxidases
Structural basis of activation of mammalian heme peroxidases
P. K. Singh, H. V. Sirohi, N. Iqbal, P. Kaur, S. Sharma and T. P. Singh*
Department of Biophysics, All India Institute of Medical Sciences, New Delhi
E-mail: [email protected]
Abstract:
The mammalian heme peroxidases, including lactoperoxidase (LPO),
myeloperoxidase (MPO), eosinophil peroxidase (EPO) and thyroid peroxidase (TPO)
utilize H2O2 to catalyze a diverse set of reactions. Three of them, MPO, EPO and LPO
are primarily found in granules of neutrophils, eosinophil leucocytes and secretions of
exocrine glands respectively. Although several substrates have been found in vitro, the
physiologically relevant substrates are believed to be chloride for MPO and thiocyanate
for LPO and EPO which are oxidized to the toxic products hypochloride and
hypothiocyanate. TPO functions in the biosynthesis of thyroid hormones by oxidizing
iodide ion. It is understood that the mature mammalian peroxidases are post
translationally modified with heme covalently linked to the protein via two ester bonds
involving conserved glutamate and as residues. In LPO, the two ester covalent linkages
are formed involving Glu258 and Asp108 with 1-methyl and 5-methyl groups of heme
moiety respectively. However, the structures of LPO and MPO in the unbound states
have revealed that the ester covalent linkage between Glu258 and heme moiety is not
present in all the protein molecules. In such structures, Glu258 occupies two positions,
one corresponds to normal ester covalent linkage while in the second position, the
side chain of Glu258 is located in the distal heme cavity whereby it blocks the substrate
binding site leading to its inactivation. As H2O2 acts as an electron donor and interacts
with the heme iron throgh its binding in the distal heme cavity. As a result, it is able to
push the side chain of Glu258 to coincide with the first position to form an ester linkage
with heme moiety thus vacating the space of the distal heme cavity for the binding of
substrates. This shows that the native mammalian heme peroxidases exist as inactive
proteins. Upon exposure to H2O2, these enzymes are activated in which H2O2 is
involved in two step process, first by structurally clearing the distal heme cavity for the
binding of substrates and then donating the electron for the chemical action to take
place.

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